Ulster Unionist leadership candidate Mike Nesbitt has admitted that he cannot deliver a referendum on changes to Stormont’s structures without support from the other parties.

The Strangford MLA also acknowledged that there will be no referendum on an official opposition before the next Assembly election.

Mr Nesbitt is running for the leadership against deputy leader John McCallister, after Tom Elliott announced he would be stepping down from the position at the party's AGM on March 31.

However, Mr Nesbitt said formal opposition was “inevitable” and that a lot had changed since the 1998 Good Friday Agreement.

He said while the 1998 referendum was carried by a significant majority of people (71%), the DUP always raised doubts about the level of support within unionism.

“Since then, the DUP and Sinn Fein negotiated fundamental changes to that Agreement, not least in how the First Minister and deputy First Minister are chosen,” Mr Nesbitt said.

“This important constitutional point was never referred to the people for approval.”

However, any referendum on structural changes appears highly unlikely, with the SDLP and Sinn Fein both opposed to such a move — making consensus across the political divide practically impossible to achieve. But Mr Nesbitt denied he was diverting attention away from the debate on unofficial opposition because he detects support for it.

“I want the DUP to be the opposition,” he said. “What John is calling for is to give up the power we have and I just don’t see an appetite for it.”

On Sunday night, DUP MLA Simon Hamilton hit back at Mr Nesbitt’s “cheap shots” and said his party has a consistent record of seeking to change the structures of government at Stormont.

Meanwhile, Mr McCallister has said he to wants to see the UUP form an unofficial opposition on April 2 if he is elected leader.

If the South Down MLA wins and take the UUP into unofficial opposition, the party would lose its sole ministerial position — although committee chairs and deputy chairs would remain.

“We will be an opposition and hold this administration to account and provide the people of Northern Ireland an alternative at the next Assembly elections,” Mr McCallister said. “It is actually healthy to have an opposition, every type of democratic system in the world has an opposition or checks and balances on power and that’s what we need.”

Mr Nesbitt also spoke of an “exciting mood for change” among party membership and stressed that the leadership contest should not be turned into a single issue debate.